European earthquakes: new image of the sub-seafloor

12th September 2019

iStock/Duncan Walker

New investigations are providing us with an insight on great European earthquakes with implications for the Earth’s long cycles.

On the 1st of November 1755, a major earthquake struck the Iberian Peninsula. 40 minutes after the main shock, a giant tsunami rose the Tagus River flooding the ruined city of Lisbon. Together, the earthquake, the tsunami and the fires killed tens of thousands of people. The event become known as the ‘Great Lisbon Earthquake of 1755’. With an estimated magnitude of 8.7, it was, historically, the biggest earthquake ever to hit Europe. The incident provoked the interest of many thinkers of the Enlightenment era. Philosophers such as Voltaire, Rosseau and Kant become interested in the event and wrote several texts about it. In particular, Immanuel Kant wrote in 1756 three treaties on the causes of earthquakes. For some, this is considered the inauguration of modern seismology or even of the broader field of earth sciences.

The 1755 earthquake was unique in many ways, but it was not an isolated event. In 1969, precisely 50 years ago, another major earthquake with a magnitude 7.9 hit the offshore of Iberia. This alerted scientists to the idea that something unusual could be happening off the coasts of Portugal. Also at this time, another unique moment in the history of earth sciences was happening: the birth of the theory of plate tectonics.

Plate tectonics is the modern unifying theory of solid earth sciences, describing the geological processes in one integrated framework. Plate tectonics describes that the planet’s outer shell is fragmented in several plates that move in relation to each other. Many fundamental geological processes occur at plate boundaries. These include earthquakes, volcanoes and the formation of a large number mineral deposits.

Continental margin types – Pacific and Atlantic

In 1969, geoscientists already knew that there were two types of continental margins. Pacific-type margins that correspond to plate boundaries and host most of the Earth’s volcanoes and high magnitude earthquakes (the famous ring of fire), and Atlantic-type margins that do not correspond to plate boundaries and generally do not house major earthquakes nor volcanoes. The margin of Iberia is, however, particular in this regard.

While the West margin of Portugal corresponds to a classical Atlantic-type margin, its southern region is located near the Azores-Gibraltar Fracture Zone, a transform segment of the Africa-Eurasia plate boundary. Notwithstanding, very high magnitude earthquakes are uncommon in the Atlantic and therefore, several scientists headed to this region to investigate these outstanding seismic events.

The first marine geology works in the Southwest Iberian margin began in the early 70’s. From the very beginning, geoscientist understood that something very peculiar was occurring here. It was proposed that in offshore of Portugal an Atlantic-type margin was transforming in a Pacific-type margin. If this was the case, it would correspond to a unique place in the Atlantic where a new plate boundary was just forming, where one plate was starting to sink beneath another. However, at the time the available data from the seafloor was scarce and these ideas took several years to mature.

Mapping the seafloor of Southwest Iberia

It was only during the 90’s and the beginning of the new Millennium that numerous initiatives started to emerge with the objective of mapping the seafloor of Southwest Iberia. This was a great effort lead by many international and multidisciplinary teams that allowed collecting seismic data and high-resolution bathymetry, always with the objective of finding the source of the Great Lisbon Earthquake of 1755. However, it was soon realised that this was a very complex region with the seafloor deformation being spread over a large area with a complex network of active tectonic faults that controlled the uplift of massive submarine sea mountains

However, there was something enigmatic; the 1969 earthquake, which had already been registered by land seismometers, had its source in a totally flat area of the Horseshoe Abyssal Plain, far from any major mapped tectonic faults. Furthermore, the illusive source of the Great Lisbon Earthquake of 1755 was never found, even though several possible locations have been proposed over the years.

In 2007, within the scope of the European Project NEAREST several seismometers were deployed in the seafloor for nearly one year. The data, together with data from land stations, allowed us to obtain a new image of the sub-seafloor.

Using this information, we have recently identified an anomaly in the mantle, at depths up to 250 km that seems to correspond to the peeling of the lower portion of a tectonic plate that appears to start sinking into the mantle. This anomaly, located precisely below the epicenter of the 1969 earthquake, may correspond to the seed that in time, may cause the transformation of this Atlantic-type margin in a Pacific-type one. Furthermore, this deep and buried structure, has the characteristics to be the source of some of the high magnitude earthquakes that have been affecting the Southwest Iberian Margin. Further work will be needed to improve our understanding of this region, which will certainly have an impact in the evaluation of the seismogenic and tsunamigenic risk to which Western Europe is exposed.

Major implications for Earth long cycles

The process of transformation of an Atlantic-type margin in a Pacific-type margin has also major implications for the Earth long cycles. It is known that the Atlantic was formed as the result of the break-up of the super-continent Pangaea. We also know that Pangea was not the first supercontinent to have existed on Earth and that supercontinents form semi-periodically in what is known as the supercontinent cycle. This is because oceans have a life cycle; they open, spread and at a certain point start to shrink and close. For a spreading ocean such as the Atlantic to start closing, its Atlantic-type margins must transform in Pacific-type margins. This is precisely what seems to be happening in the coasts of Iberia. This means that the Atlantic Ocean may be reaching its old age and will soon start to close. In time, the Americas may reunite with Europe to form a new supercontinent.

We are only now starting to grasp the fascinating and largely unknown processes that occur in the deep ocean. The improvement and development of new technologies is essentially to map and monitor the sub-seafloor. New discoveries will not only help us to identify and understand the natural hazards to which we are exposed but will also allow us to comprehend the large-scale dynamics of the unique planet in which we are living. Events such as the Great Lisbon Earthquake of 1755 will happen again and are part of the natural dynamics of our planet. It is up to us to make sure that we are prepared for these outstanding events.